The present invention is directed towards a process for the preparation of very highly concentrated hydrogen peroxide of concentration cP from aqueous hydrogen peroxide of concentration CE, CE being at least 80 wt. % and CP being greater than CE, comprising continuous suspension crystallization with subsequent washing of the hydrogen peroxide that has crystallized out. The process is directed especially towards the preparation of over 98 wt. % hydrogen peroxide from approximately 90 wt. % hydrogen peroxide, as well as at least 99.9 wt. % hydrogen peroxide of very high purity obtainable according to the process.
In the known processes for the preparation of hydrogen peroxide, such as the anthraquinone cyclic process, electrolytic processes and methods of direct synthesis, hydrogen peroxide is obtained in the form of an aqueous solution. In order to convert such solutions into marketable products, the preparation is usually followed by concentration of the solution by distillation. By means of distillation it is possible to obtain aqueous hydrogen peroxide solutions having a content of up to approximately 90 wt. % hydrogen peroxide. Cost and safety considerations stand in the way of further concentration by distillation.
More highly concentrated hydrogen peroxide, that is to say products having an H2O2 content in the range of from equal to/greater than 90 wt. % to approximately 98 wt. %, are increasingly gaining importance commercially owing to their high energy content and their high purity. It is known that hydrogen peroxide can be concentrated by repeated recrystallization. Fundamental aspects regarding the crystallization of hydrogen peroxide, as well as the solid-liquid phase diagram of the hydrogen peroxide/water system, are known from the reference book xe2x80x9cHydrogen Peroxidexe2x80x9d by Walter C. Schumb et al., Reinhold Publishing Corp. (1955), p. 210-220. According to that book, a prominent property of highly concentrated aqueous hydrogen peroxide is the tendency to extreme supercooling, which makes concentration by crystallization more difficult. According to Schumb et al. (page 215), crystal formation is not affected in a reproducible manner either by stirring or scraping or by the cooling rate. However, freezing of supercooled highly concentrated hydrogen peroxide is induced by seeding with hydrogen peroxide crystals. The low efficiency of fractional batch crystallization is regarded as being a disadvantage of concentration by crystallization (Schumb et al., p. 215), since an increase in concentration of only approximately 2% and never more than 4% is achieved per crystallization stage. That problem is evidently the result of the high boundary solubility, that is to say mixed crystal formation, of water in crystallized hydrogen peroxide, which is also shown in the phase diagram (Schumb, page 211). Because of that boundary solubility, it has hitherto not been possible, according to the experience of the present inventors, to obtain hydrogen peroxide having a content of at least 99.9 wt. % by fractional crystallization.
DE-PS 10 41 479 teaches a continuous process for the concentration of aqueous hydrogen peroxide solutions, which process also allows very highly concentrated hydrogen peroxide to be obtained. The apparatus for concentration comprises a crystallizing vessel and a rectifying column arranged directly beneath it. The hydrogen peroxide starting material is supercooled in the crystallizing vessel until there forms a 2-phase system consisting of a solid crystal phase and a liquid mother liquor, which phases differ according to the equilibrium with regard to the hydrogen peroxide concentration. The temperature along the rectifying column is so controlled that it increases slowly from the upper to the lower end. Owing to the higher density of the crystallized hydrogen peroxide, the crystals sink downwards in the rectifying column, a solid-liquid exchange of material taking place until the respective equilibrium has been reached and the H2O2 concentration in the crystals increasing towards the bottom; the H2O2 concentration in the mother liquor falls towards the top. Because there is only a slight difference in density between the crystallized hydrogen peroxide and the hydrogen peroxide mother liquor, the process is characterized by a low space-time yield. A further very considerable disadvantage that prevents the prior-known process from being applied on an industrial scale consists in scale-up problems (application of the process on an industrial scale): as the diameter of the rectifying column increases, unforeseeable back-mixing occurs, as a result of which both the function is impaired and the process risk is increased considerably (see Wellinghoff et al. in Chem.-Ing.-Tech. 63 (1991), 881-882).
The rectifying column according to DE-PS 10 41 479 is in principle operated as a gravity washing column. In the lowermost portion of the column, some of the crystals melt again and the melt thus acts as the washing medium. In such washing columns, working on the countercurrent principle, solid-liquid separation and further purification of the solid take place in one apparatus, but the scale-up problems remain, even when the suspension crystallization according to the invention described below is carried out.
Although the above-indicated scale-up problems can be avoided, as has been described in the as yet unpublished patent application DE 100 54 742.7, by means of a layer crystallization process with a subsequent sweating operation, the advantage of the simple separation of solid and liquid that can be carried out without problems even on a large scale is, however, offset by the disadvantages of discontinuous, multi-stage operation and the higher energy requirement.
Accordingly, an object of the present invention is to provide a continuous process for the preparation of very highly concentrated hydrogen peroxide from at least 80 wt. % aqueous hydrogen peroxide, which process can be carried out without problems even on an industrial scale.
According to a further object of the invention it is desired to obtain over 98 wt. %, preferably at least 99.9 wt. %, hydrogen peroxide from approximately 90 wt. % hydrogen peroxide in one step.
A still further object, in addition to concentrating and hence obtaining substantially anhydrous hydrogen peroxide, is directed towards lowering the content of impurities, such as organic carbon and conventional stabilizers. Conventional impurities are process-dependent secondary constituents, and stabilizers added during/after the preparation of the H2O2 starting material to be concentrated.
The above and other objects can be achieved in a simple manner and with an unforeseeably high degree of efficiency by means of the process according to the invention.
Accordingly, the invention provides a process for the continuous preparation of very highly concentrated hydrogen peroxide of concentration CP from aqueous hydrogen peroxide of concentration CE, CE being at least 80 wt. % and CP being greater than CE, comprising suspension crystallizing aqueous hydrogen peroxide of concentration CE and after-treating the resulting hydrogen peroxide crystals contained in the suspension. In further detail, the process is characterised in that the after-treatment takes the form of countercurrent washing in a hydraulic or mechanical washing column with a packed crystal bed, and molten hydrogen peroxide of concentration CP is used as the washing medium. The dependent claims are directed towards preferred embodiments of the process according to the invention.
The process is especially suitable for concentrating aqueous hydrogen peroxide having a concentration CE in the range from 85 to 95 wt. %, especially from 88 to 92 wt. %, in a single step, preferably to a concentration CP of over 98 wt. %, but especially equal to or greater than 99.9 wt. %. In addition to concentration of the H2O2 starting material, the content of impurities is at the same time lowered to surprisingly low values. For example, the TOC content (total organic carbon) of, for example, from 40 to 50 mg/kg in the H2O2 starting material can be reduced in one step to a value below approximately 3 mg/l of H2O2 product. The content of other secondary constituents and stabilizers, such as phosphate, tin and nitrate, can also be reduced to values below the detection limit of conventional analytical methods, generally to values below 3 mg in each case, especially below 1 mg/kg of H2O2 product.
Accordingly, the present invention also provides hydrogen peroxide having a concentration of from 99.9 to 100 wt. % and a content of TOC, nitrate, phosphate, nickel and tin of less than 4 mg/l in each case. The 99.9 to 100 wt. % hydrogen peroxide preferably contains less than 1 mg of Ni and Sn per liter of H2O2 and less than 2 mg of phosphate per liter of H2O2.
The substantially anhydrous hydrogen peroxide obtainable according to the invention and having an extraordinarily low content of impurities can be used, for example, as a motor fuel, as an oxidizing agent or for electronics purposes, such as the preparation of solutions for the treatment of electronic components. If required, it is also possible to add to the very highly concentrated and very pure H2O2 an effective amount of one or more auxiliary substances directed towards the particular application or/and of stabilizers known per se or of a stabilizer combination. Suitable stabilizers are, for example, tin compounds, phosphates, di- and triphosphates, phosphonates and radical acceptors. The very high purity of the H2O2 product that can be achieved by means of the process according to the invention is, therefore, a way of replacing a stabilizer combination contained in the H2O2 starting material by other auxiliary substances/active ingredients in the H2O2 product.